The demand for remote access to information sources and data retrieval, as evidenced by the success of services such as the World Wide Web, is a driving force for high-speed network access technologies. Today's telephone network offers standard voice services over a 4 kHz bandwidth. Traditional analog modem standards generally assume that both ends of a modem communication session have an analog connection to the Public Switched Telephone Network (PSTN). Because data signals are typically converted from digital to analog when transmitted towards the PSTN and then from analog to digital when received from the PSTN, data rates may be limited to 33.6 kbps as defined in the V.34 transmission recommendation developed by the International Telecommunications Union (ITU).
The need for an analog modem can be eliminated, however, by using the Basic Rate Interface (BRI) of the Integrated Services Digital Network (ISDN). A BRI offers end-to-end digital connectivity at an aggregate data rate of 160 kbps, which is comprised of two 64 kbps B channels, a 16 kbps D channel, and a separate maintenance channel. ISDN can offer comfortable data rates for Internet access, telecommuting, remote education services, and some forms of video conferencing. ISDN deployment, however, has been very slow due at least in part to the substantial investment for new equipment. Because ISDN presently is not very pervasive in the PSTN, the network providers have typically tarriffed ISDN services at relatively high rates, which may be ultimately passed on to the ISDN subscribers. In addition to the high service costs, subscribers must generally purchase or lease network termination equipment to access the ISDN.
While most subscribers do not enjoy end-to-end digital connectivity through the PSTN, the PSTN is nevertheless mostly digital. Typically, the only analog portion of the PSTN is the phone line or local loop that connects a subscriber or client modem (e.g., an individual subscriber in a home, office, or hotel) to the telephone company's Central Office (CO). In recent years, local telephone companies have been replacing portions of their original analog networks with digital switching equipment. Nevertheless, the connection between the home and the CO generally has been the slowest to change to digital as discussed in the foregoing with respect to ISDN BRI service.
A recent data transmission recommendation issued by the ITU, known as V.90, takes advantage of the digital conversions that have been made in the PSTN. By viewing the PSTN as a digital network, V.90 technology is able to accelerate data downstream from the Internet or other information source to a subscriber's computer at data rates of up to 56 kbps, even when the subscriber is connected to the PSTN via an analog local loop.
To understand how the V.90 recommendation achieves this higher data rate, it may be helpful to briefly review the operation of V.34 analog modems. V.34 modems are optimized for the situation where both ends of a communication session are connected to the PSTN by analog lines. Even though most of the PSTN is digital, V.34 modems treat the network as if it were entirely analog. Moreover, the V.34 recommendation assumes that both ends of the communication session suffer impairment due to quantization noise introduced by analog-to-digital converters. That is, the analog signals transmitted from the V.34 modems are sampled at 8000 times per second by a codec upon reaching the PSTN, with each sample being represented or quantized by an eight-bit pulse code modulation (PCM) codeword. The codec uses 256, non-uniformly spaced, PCM quantization levels defined according to either the μ-law or A-law companding standard (i.e. the ITU G.711 Recommendation).
Because the analog waveforms are continuous and the binary PCM codewords are discrete, the digits that are sent across the PSTN can only approximate the original analog waveform. The difference between the original analog waveform and the reconstructed quantized waveform is called quantization noise, which can limit the modem data rate.
While quantization noise may limit a V.34 communication session to 33.6 kbps, it nevertheless affects only analog-to-digital conversions. The V.90 standard relies on the lack of analog-to-digital conversions in the downstream path, outside of the conversion made at the subscriber's modem, to enable transmission at 56 kbps.
The general environment for which the V.90 standard was developed is depicted in FIG. 1. An Internet Service Provider (ISP) 22 is connected to a subscriber's computer 24 via a V.90 digital server modem 26, through the PSTN 28 via digital trunks (e.g., T1, E1 and/or ISDN Primary Rate Interface (PRI) connections), through a central office switch 32, and finally through an analog loop to the client modem 34. The central office switch 32 is drawn outside of the PSTN 28 to better illustrate the connection of the subscriber's computer 24 and modem 34 into the PSTN 28. It should be understood that the central office 32 generally is, in fact, a part of the PSTN 28. Operation of a communication session between the subscriber 24 and an ISP 22 is best described with reference to the more detailed block diagram of FIG. 2.
Referring to FIG. 2, transmission from the server modem 26 to the client modem 34 will be described first. The information to be transmitted is first encoded using only the 256 PCM codewords used by the digital switching and transmission equipment in the PSTN 28. These PCM codewords are transmitted towards the PSTN by the PCM transmitter 36 where they are received by a network codec.
The PCM data is then transmitted through the PSTN 28 until reaching the central office 32 to which the client modem 34 is connected. Before transmitting the PCM data to the client modem 34, the data is converted from its current form as either μ-law or A-law companded PCM codewords to Pulse Amplitude Modulated (PAM) voltages by the codec expander (digital-to-analog (D/A) converter) 38. These PAM voltage levels are processed by a central office hybrid 42 where the unidirectional signal received from the codec expander 38 is transmitted towards the client modem 34 as part of a bidirectional signal. A second hybrid 44 at the subscriber's analog telephone connection converts the bidirectional signal back into a pair of unidirectional signals.
Finally, the analog signal from the hybrid 44 is converted into digital PAM samples by an analog-to-digital (A/D) converter 46, which are received and decoded by the PAM receiver 48. Note that for transmission to succeed effectively at 56 kbps, there should be only a single digital-to-analog conversion and subsequent analog-to-digital conversion between the server modem 26 and the client modem 34. Recall that analog-to-digital conversions in the PSTN 28 can introduce quantization noise, which may limit the data rate as discussed hereinbefore. The A/D converter 46 at the client modem 34, however, may have a higher resolution than the A/D converters used in the analog portion of the PSTN 28 (e.g. 16 bits versus 8 bits), which results in less quantization noise. Moreover, the PAM receiver 48 preferably is in synchronization with the 8 kHz network clock to properly decode the digital PAM samples.
Transmission from the client modem 34 to the server modem 26 follows the V.34 data transmission standard. That is, the client modem 34 includes a V.34 transmitter 52 and a D/A converter 54 that encode and modulate the digital data to be sent using techniques such as Quadrature Amplitude Modulation (QAM). The hybrid 44 converts the unidirectional signal from the digital-to-analog converter 54 into a bidirectional signal that is transmitted to the central office 32. Once the signal is received at the central office 32, the central office hybrid 42 converts the bidirectional signal into a unidirectional signal that is provided to the central office codec. This unidirectional, analog signal is converted into either μ-law or A-law companded PCM codewords by the codec compressor (AID converter) 56, which are then transmitted through the PSTN 28 until reaching the server modem 26. The server modem 26 includes a conventional V.34 receiver 58 for demodulating and decoding the data sent by the V.34 transmitter 52 in the client modem 34. Thus, data is transferred from the client modem 34 to the server modem 26 at data rates of up to 33.6 kbps as provided for in the V.34 standard.
Thus, the V.90 standard offers increased data rates (e.g., data rates up to 56 kbps) in the downstream direction from a server to a subscriber or client. Upstream communication still generally takes place at conventional data rates as provided for in the V.34 standard. Nevertheless, this asymmetry is particularly well suited for Internet access. For example, when accessing the Internet, high bandwidth generally is most useful when downloading large text, video, and audio files to a subscriber's computer. Using V.90, these data transfers can be made at up to 56 kbps. On the other hand, traffic flow from the subscriber to an ISP generally includes mainly keystroke and mouse commands, which are readily handled by the conventional rates provided by V.34.
As described above, the digital portion of the PSTN 28 transmits information using eight-bit PCM codewords at a frequency of 8000 Hz. Thus, it would appear that downstream transmission should take place at 64 kbps rather than 56 kbps as defined by the V.90 standard. While 64 kbps is a theoretical maximum, several factors may prevent actual transmission rates from reaching this ideal rate. First, even though the problem of quantization error can be substantially eliminated by using PCM encoding and PAM for transmission, additional noise in the network or at the subscriber premises, such as non-linear distortion and crosstalk, can limit the maximum data rate. Furthermore, the μ-law or A-law companding techniques generally do not use uniform PAM voltage levels for defining the PCM codewords. The PCM codewords representing very low levels of sound have PAM voltage levels spaced close together. Noisy transmission facilities can prevent these PAM voltage levels from being distinguished from one another thereby causing loss of data. Accordingly, to provide greater separation between the PAM voltages used for transmission, not all of the 256 PCM codewords may be used.
It is generally known that, assuming a convolutional coding scheme, such as trellis coding, is not used, the number of symbols to transmit a certain data rate is given by Equation 1:bps=Rslog2Ns  EQ. 1 where bps is the data rate in bits per second, Rs is the symbol rate, and Ns is the number of symbols in the signaling alphabet or constellation. To transmit at 56 kbps using a symbol rate of 8000, Equation 1 can be rewritten to solve for the number of symbols required as set forth below in Equation 2:Ns=256000/8000=128  EQ. 2 Thus, the 128 most robust codewords of the 256 available PCM codewords generally are chosen for transmission as part of the V.90 standard.
Successful operation of a V.90 receiver may depend on an accurate identification of the reference PAM signaling levels that are often called the signaling alphabet or the signal constellation. The digital samples that are filtered by a decision feedback equalizer are provided to a slicer/detector where the samples are compared against the signaling alphabet. A determination is made with regard to which member of the alphabet or which point in the constellation the digital sample falls closest to. Once the alphabet member is identified, the PCM code word corresponding to that alphabet member is selected as the symbol transmitted for that digital sample.
While a set of ideal signaling levels can be defined for the signaling alphabet, the effective alphabet typically will deviate from these ideal levels because of underlying digital impairments resulting from Robbed Bit Signaling (RBS) and/or digital attenuation PADs. RBS is a mechanism utilized in the digital transport system, such as a T1 trunk, for signal control and status information between network equipment. PAD is similarly found in the digital transport system for the purpose of adjusting signal levels for different analog and digital equipment. Since these impairments will likely be chronic throughout the communication session, it may be more efficient for the modem to learn a new signaling alphabet that takes these impairments into account.
Accordingly, the V.90 standard specifies that during Phase 3 of the startup procedure that is carried out after establishing a dialed connection between the client and server modems, digital impairment learning will take place. During digital impairment learning, a plurality of sets of DIL signals, each corresponding to a set of PCM signals, is repeatedly transmitted from a server modem to a client modem during a corresponding plurality of DIL intervals, also referred to as framing intervals. For example, six DIL intervals may be provided during which all or a selected subset of the PCM levels for the constellation are transmitted. The plurality of DIL intervals may be repeated until the RBS and PAD digital impairments are identified. The PAD and RBS digital impairments so identified are then used in the Phase 4 final training procedures for the V.90 modem.
Unfortunately, the identification of RBS and PAD digital impairments may be difficult because of the many types of RBS and the many levels of PAD digital impairments that may be present in a telephone network. RBS and PAD identification also may be difficult due to the combinations of one or more PADs and/or RBS that may be present in a given network.
For example, RBS can manifest itself when the Least Significant Bit (LSB) of a PCM code word in a particular DIL interval is forced to a one. This operation has the effect of collapsing neighboring PCM code words with even and odd values into the odd value PCM code word. Other types of RBS variations are possible, and different DIL intervals may be subject to different types of RBS.
The effect of PADs generally is present in all six DIL intervals. A PAD also can result in multiple PCM code words collapsing into a single code word. Although this may not cause a problem for voice transmission, it may produce great difficulty for data transmission. PADs generally are not standardized and several quantization rules can be used for implementing a given PAD attenuation.
Accordingly, it is desirable to provide improved systems, methods and/or computer program products for identifying RBS and PAD digital impairments in the DIL signals that are repeatedly transmitted from a server modem to a client modem during a corresponding plurality of DIL intervals.